The present invention relates to quantitative analytic sensors. More specifically the present invention relates to a sensor that uses an electrode response to measure the concentration of chloramine in a solution.
Chloramine is often used in the treatment of water. While chloramine is generally not as powerful a disinfectant as chlorine, it is often used instead of chlorine because it persists longer and provides a number of other benefits. Thus, sensing chloramine provides useful information for such treatment systems as well as any other system where chloramine is used.
Amperometric sensors are generally known. In such sensors, a species of interest reacts electrically to generate an electrical response that is measured in the form of current flow. One example of a chloramine amperometric sensor is the Model 499A DO-54-99 (SQ6684) available from Emerson Process Management, Rosemount Analytical Division, of Irvine Calif.
Development of embodiments of the present invention is due, at least in part, to a recognition of limitations of current state of the art chloramine amperometric sensors. For example, current sensors generally use a sensing electrode that consists of a solid metallic disc or other shape that is generally a noble metal. The chloramine diffuses across a gas-permeable membrane, such as polytetrafluoroethylene (PTFE) and enters an electrolytic solution. The chloramine then reduces a second species such as I− into I2. The reduced second species, such as I2, then obtains electrons from the sensing electrode to generate a current that is related to the quantity of chloramine. However, sensor linearity begins to drop off for higher concentrations of chloramine, about 2 ppm. It is believed that conventional sensors limit the access of the second species, such as I− to sensing electrode (cathode) due to the geometry of the sensing electrode. Another problem with current amperometric sensors for chloramine sensing is due to the activity of dissolved oxygen. If oxygen is dissolved in the chloramine containing specimen, the dissolved oxygen will reduce at a level similar to the chloramine, thus “clouding” the measured chloramine response.